The Eye of the Beholder

After three weeks of only sparse sightings of dragonflies, this past week in Terra Nova has been absolutely flooded with these graceful creatures. A few members of the family Aeshnidae seem to be of a particular abundance along the peripheries of the many ponds that punctuate Terra Nova.Green Darners are (relatively) massive with their 3-inch wingspans – they are magnificent flyers, darting through the air with unparalleled precision and speed. Their disproportionately huge eyes process their local environment at a rate I cannot even comprehend. Catching the Green Darner with our nets is typically a challenge given their agility, though with such a large number of them soaring through the air it’s hard not to have one glide into our clutches.

A Shadow Darner, Aeshna umbrosa, found in Ochre Hill Forest in Terra Nova National Park, posing for a picture.

I’ve included photos of two of the dragonflies we had encountered while collecting in Terra Nova. In both the photo of the Green Darner and of the Shadow Darner, I am immediately drawn to the spectacular eyes these two creatures possess. Each compound eye is subdivided into approximately 30 thousand individual light-processing units called ommatidia. Each ommatidium contains a cluster of light sensitive cells called photoreceptors, which are responsible for providing a direct visual bridge from the external world of light to the brain. The sum of the 30 thousand individual light processing units weave together an image of the dragonfly’s surroundings. What the resulting final image truly ‘looks’ like to the dragonfly is beyond human comprehension, though this image undoubtedly helps drive the dragonfly’s immense ability to fly.

With many who study biology, there is often a common revelation or conclusion to be had: we humans are woefully inept in our sensory abilities compared to some of evolution’s other offerings. The dragonfly, not unlike the mantis shrimp, has the ability to process ultraviolet light in addition to the typical red, greenand bluewavelengths that we humans are limited to. The UV light is believed to provide a backdrop against the sky that allows for better resolution and detection of desired prey. In addition to a broader spectrum of visible light for the dragonfly, they also have the benefit of a broader field of view. With their compound eyes bulging outwards into the air, these aerial predators see a lot more than you or I at any given time.

The Strange Case of the Caddisfly

The Strange Case of the Caddisfly

When we were finishing retrieving an aquatic bottle trap from Blue Hill today we happened to find some tubes of carefully interwoven pine needles. While these types of things would normally go unnoticed by being inconspicuously small and blending in with the neighboring debris that has been swept to shore, what we were actually looking at was the discarded casing of a caddisfly larva (from the order Trichoptera).

An abandoned caddisfly larva case

Caddisfly larva typically spin webs from glands in their mouth and, in case building species, will collect surrounding debris and other bits of nature to make their home. They keep both ends of their case open so that they can have a continuous flow of water (going from posterior end to anterior end) in order to breathe. To make sure that they don’t fall out they will hold their home in place by a pair of hooked prolegs at the end of their abdomen. Once they have matured they will bite through the case and abandon their homes to join the world of flight. Caddisflies are adults for merely a week or two of their year-long lives, and often they do not feed when they’re mature. The ability of caddisflies to make their homes from surrounding debris has been harnessed by bug-savvy jewelers to create one-of-a-kind pieces that you can wear.

A jewel encrusted caddisfly larva from www.cabinetmagazine.org

Other caddisfly larva will create silken nets to filter and funnel water-borne food to their mouth, and some do not make silk cocoon-like structures at all and are free-living in the water. Learn more about other water-linked species in Forest’s blog on the fascinating lives of dragonflies!

Greetings from Terra Nova National Park!

Jill

P.S. The Barcode of Life Initiative has a Trichoptera campaign! Check it out at www.trichopterabol.org